This invention pertains to the volumetric measurement of flow and, particularly, a flow prover useful in periodically calibrating a continuous flow meter in a pipeline without interrupting the flow of fluid. The flow prover of this invention falls generally into that class of flow provers characterized by the measurement of the movement of a piston travelling through a cylinder wherein the piston has a plurality of seals which form a fluid barrier in the annular space between the piston and the cylinder. This invention pertains specifically to a compact flow prover having means for monitoring the seal integrity.
U.S. Pat. No. 3,738,153 to Simmons discloses a seal monitor for a flow prover using resilient balls. A hydraulic cylinder, used to open and close passages for the balls, has a seal monitor that detects a pressure difference between the surrounding fluid and the fluid in the region between a passage and the cylinder piston head. However, in Simmons the piston head does not slide sealingly along a testing cylinder and therefore the seals can be monitored while the piston head is stationary.
A flow having means for monitoring the seal integrity of its piston is disclosed in U.S. Pat. No. 4,372,147 to Waugh. Waugh discloses a flow meter prover having a measuring conduit coaxially mounted within on outer housing which has fluid apertures adjacent the upstream and downstream ends of the measuring conduit, a piston mounted within the conduit, an actuating rod axially projecting from the downstream side of the piston with the free end of the rod extending through the downsteam end of the outer housing, and piston detection switches spaced along the length of the measuring conduit. The piston is provided with two seals each encircling the perimeter of the piston to form an annular cavity between the seals. A flexible tube, helically wrapped about the rod, has one end connected to a passage which is in fluid communication with the annular cavity, and the other end of the flexible tube is connected to the exterior of the apparatus. During the operation of the prover, fluid leakage past either of the seals causes fluid flow that can be detected by monitoring the pressure in the flexible tubing by means of the exterior connection to the flexible tubing.
It is believed that there are several disadvantages associated with devices of the type disclosed in Waugh. One potential disadvantage is that the flexible tubing is cycled with the reciprocating piston, and over a period of time this may ultimately lead to a failure of the flexible tube. Another potential disadvantage is that the differential pressure between the flexible tubing and the apparatus fluid may result in collapse or rupture of the tubing. Still another potential disadvantage is that a pressure source or a bleed system may be required. In either case, seal monitoring is complicated by the movement of the seals past the fluid apertures in the measuring conduit. When the integrity of the seals is monitored dynamically during a proving run, a control system to rapidly increase pressure or bleed the flexible tubing after it moves past the fluid apertures may be required so that a decrease or increase in pressure in the flexible tubing may be observed before the proving run is completed. Moreover, it is believed that during the pressuring or bleeding, fluid leakage past the seals may not be detected.
It is a feature of this invention to provide a flow prover having means for monitoring the seal integrity of the flow prover piston which overcomes many of the disadvantages associated with known devices.